fix(sessions): use async iteration for VertexAiSessionService.list_sessions pagination

Merge https://github.com/google/adk-python/pull/4435

### Link to Issue or Description of Change

- Closes: #4302

**Problem:**

`VertexAiSessionService.list_sessions()` only returns the first ~100 sessions. The `sessions_iterator` from `api_client.agent_engines.sessions.list()` is an `AsyncPager` — it implements `__aiter__`/`__anext__` for fetching subsequent pages, but the code uses a plain `for` loop which only calls `__iter__`/`__next__`, so it never fetches beyond the first page.

**Solution:**

Changed `for api_session in sessions_iterator` to `async for api_session in sessions_iterator` so the `AsyncPager` actually paginates. Updated the test mock to return an `AsyncIterableList` (supports both sync and async iteration) instead of a bare list, so the tests properly simulate real `AsyncPager` behaviour.

### Testing Plan

**Unit Tests:**

```
$ pytest tests/unittests/sessions/
115 passed, 1 warning in 2.25s
```

The existing `test_list_sessions`, `test_list_sessions_with_pagination`, and `test_list_sessions_all_users` all continue to pass with the updated mock.

Co-authored-by: Liang Wu <wuliang@google.com>
COPYBARA_INTEGRATE_REVIEW=https://github.com/google/adk-python/pull/4435 from anmolg1997:fix/vertex-ai-session-service-pagination 14c71b607ecbf2215f4b9ba6eb4b0ff6b9eaf740
PiperOrigin-RevId: 868466166

src/google/adk/sessions/vertex_ai_session_service.py

@@ -215,7 +215,7 @@ class VertexAiSessionService(BaseSessionService):
           config=config,
       )
 
-      for api_session in sessions_iterator:
+      async for api_session in sessions_iterator:
         sessions.append(
             Session(
                 app_name=app_name,

tests/unittests/sessions/test_vertex_ai_session_service.py

@@ -300,20 +300,20 @@ class MockAsyncClient:
     if user_id_match:
       user_id = user_id_match.group(1)
       if user_id == 'user_with_pages':
-        return [
+        return to_async_iterator([
             _convert_to_object(MOCK_SESSION_JSON_PAGE1),
             _convert_to_object(MOCK_SESSION_JSON_PAGE2),
-        ]
-      return [
+        ])
+      return to_async_iterator([
           _convert_to_object(session)
           for session in self.session_dict.values()
           if session['user_id'] == user_id
-      ]
+      ])
 
     # No user filter, return all sessions
-    return [
-        _convert_to_object(session) for session in self.session_dict.values()
-    ]
+    return to_async_iterator(
+        [_convert_to_object(session) for session in self.session_dict.values()]
+    )
 
   async def _delete_session(self, name: str):
     session_id = name.split('/')[-1]